Project Summary/Abstract Sarcopenia is a devastating skeletal muscle condition that occurs in advanced age and due to various chronic conditions. Despite the widespread prevalence of sarcopenia there are no treatment options and the mechanisms underlying this process are not completely understood. Hallmark characteristics of skeletal muscle aging are a reduction in myofiber size and number, dysregulated muscle function, and an increased incidence of fibrosis. An additional process altered during aging is the activity of muscle stem cells (MuSCs), which typically are quiescent but in a young environment function to fully repair damaged muscle and allow muscle to adapt to external stimuli. Although MuSCs are clearly required for development and regeneration, we still do not understand their exact role during muscle homeostasis and aging. To this end, we have generated a unique mouse model that ablates MuSC fusion activity but maintains their presence within the tissue. Our recent studies showed that MuSC fusion is required for muscle growth in a young environment, and that without MuSC fusion, pronounced development of fibrosis ensues. Moreover, our preliminary data indicate that MuSC fusion is dysregulated in aged skeletal muscle. Thus, we hypothesize that fusion of MuSCs with myofibers is necessary to maintain myofiber integrity, and dysregulation of this process leads to pathological extracellular matrix (ECM) remodeling during aging. To definitively answer these questions, we have generated numerous novel genetic reagents to manipulate both MuSC fusion and fibrosis in vivo. Based on these preliminary data and unique mouse models we propose to comprehensively determine the role for MuSCs and evaluate the consequences of fibrosis development during skeletal muscle aging. Specifically, we propose to: 1) elucidate the requirement of MuSC fusion for muscle adaptation during aging 2) molecularly dissect the mechanisms of fusion in aged skeletal muscle and 3) define the relationship between fibrosis and MuSC fusion during the development of sarcopenia. Successful completion of these studies will provide unique insight into the general mechanisms of MuSC-dependent muscle aging and provide new knowledge that will identify new therapeutic strategies to combat sarcopenia.